Light leakage between beams in an interferometer in metrology produces measurement results that depart periodically from the ideal—known as non-linearity.
Previously proposed non-linearity correction techniques are generally costly, and use extra hardware, such as additional beam splitters and detectors. One prior art technique for compensating for non-linearity in a homodyne interferometer involves balancing the two in-phase and quadrature signals within the interferometer for offset, gain and orthogonality. The signals are digitized, computed for imbalance, and analog electronics are used to inject offset and gain compensations. The compensation components are all contained within the interferometer itself. In a heterodyne interferometer, there are no in-phase and quadrature signals to balance. Thus, such a compensation method is not applicable to a heterodyne interferometer.